The invention pertains to a method for optimizing the operating and combustion processes of a diesel engine with common rail injection with the goal of minimizing pollutant emissions in which fuel is injected into each combustion chamber of the diesel engine by at least a main injection and a post-injection following the main injection.
Against the background of increasingly stringent emission standards (TA Luft=Technical Committee on Air), especially for stationary installations, power plants, pumping stations, and compressor installations, the lowering of pollutant values for nitrogen oxides is receiving more and more emphasis in engine development.
For example, DE 198 36 053 A1 describes a system for the automatic control and regulation of fuel injection that comprises a common rail injection system, in which the injection process is optimized to minimize pollutant emissions by dividing it into a preinjection, a main injection, and a postinjection phase. In this regard, the postinjection phase in particular makes significant exhaust gas improvement possible. The postinjection quantity is supplied by injectors after the combustion of the main injection quantity and is essentially discharged with the exhaust gas and reacts in an additional catalytic converter together with the nitrogen oxide to form harmless components.
As is well known, in electronically controlled injection systems for diesel engines, e.g., in the common rail injection system, which can provide a freely selectable injection pressure at all engine operating points, the fuel injection is controlled by a solenoid valve or piezoelectric valve. The solenoid valve is actuated by an electronic control unit, which is often referred to as the ECU (engine control unit), in such a way that the time at which the injection starts, the duration of the injection, and the quantity injected can be derived as a function of the operating parameters of the engine from an input-output map, which is stored in the ECU and is determined, for example, by an optimization method.
Thus, strategies for configuring the course of the injection process involving the use, for example, of memory-based injection systems to adapt the injection pressure and the actuation start time for each point in the engine speed-versus-load input-output map, as described by way of example in U.S. Pat. No. 6,073,608, were already regarded in the prior art as suitable means of optimization.
When an exhaust gas turbocharger is incorporated into the operating and combustion processes of the diesel engine, exhaust gas recycling is especially important as an effective means of reducing NOx levels on the air side of the diesel engine process. On the fuel side, as described above, common rail injection is an effective optimization technique. Because of the flexibility thus obtained with respect to providing the most suitable injection rate and mixing intensity at each moment in time, this optimization is possible basically for any combination of engine speed and engine load.
In addition to the goal of reducing NOx emissions, there is also the need in diesel engines with exhaust gas turbocharging to increase the charging air pressure inside the engine so that NOx emissions can be reduced on the air side. The previously known methods offer no suggestions on how to accomplish this.
According to the state of the art, the term “postinjection” means that a main injection, constituting about 90% of the amount to be injected, is followed by a smaller, secondary injection (which in the prior art is intended to occur in the subsiding phase of the combustion process). This makes it possible to reduce the emission of a pollutant (NOx or soot) or to decrease fuel consumption without changing any of the other values. The interval between the main injection and the postinjection can be varied within certain limits. If the interval is too short, however, the results are not reproducible. Conversely, if the interval is too long, the soot values in particular become worse, since the post-injected amount itself forms soot. Tests have shown that the injection of a small quantity (>10% of the main injection quantity) immediately following the main injection (about 5–15°crankshaft after the end of the main injection) gives good results in the upper load range, especially in the case of supercharged diesel engines.
Thus, the strategy of multiple injections (especially the use of postinjections) has been used until now to reduce pollutant emissions internally in the engine. Moreover, it is known that eliminating the preinjection can also lead to an improvement of pollutant emissions, especially NOx emissions.